Separatory devices have been used for many years to separate granular materials such as peanuts, coffee beans, various grains, minerals and the like according to specific weight. Such separator devices are commonly referred to in the industry as gravity separators and discharge the granular material onto a tilted, vibrating deck with a perforate bottom through which air is forced upwardly through the granular material. The upward air flow through the bed of granular material on the deck fluidizes the bed and causes the granular material to stratify according to the different specific weights of the pieces of material. The vibratory action of the deck causes the bed of granular material to shift along the deck and, when combined with the tilt of the deck, causes the different specific weight fractions in the bed of granular material to accumulate at different positions along the edges of the deck so that the accumulated fractions can be separated.
Some types of prior art separators have a relatively high tilt angle such that the granular material makes a single pass across the deck for separation. Another type of prior art separator uses a relatively low tilt angle such that the granular material remains on the deck for a significantly longer period of time and circulates over the deck surface in several passes before being removed from the deck.
This latter type separator has a stepped screen which moves the material in the bottom of the bed generally along a first direction and controls the depth of the bed of material with a discharge gate of fixed length which is opened in response to the air pressure below the deck. In this type of separator, the acceptable specific weight range fraction of the material is sought to be removed through the pneumatically operated discharged gate. Because the gate opening was of fixed length, however, it did not recognize the fact that the length and height of the accumulated acceptable specific weight range fraction of the material on the deck varied. As a result, some of the granular material in the bed outside of the acceptable specific weight range was also sometimes discharged through the pneumatically operated discharge gate. Also, the angles of the steps in the screen sometimes failed to positively move the material along the screen so that the desired degree of separation was difficult to achieve.